> > Wade Carpenter wrote:
> >
> > >Hello everyone!
> > >
> > >I have run across the problem that I need to do some high speed sampling.  High speed I mean in terms of the PIC.  Running my 876 at max speed of 20MHz, I have not too much time to do things if I want to sample at 500Khz.  And an analog signal at that.  The good part is, I only need low analog resolutions (maybe 4-bits) so I was going to build a flash ADC from discretes; 4 bit parallel ouput faster than I can handle.
> > >
> > >But I have to read continuously 128x128 pixels, store it in memory, and THEN transmit it via very slow serial to a PC (wireless!).
> > >
> > >Now my PIC has 368 bytes of RAM - slight conflict of interest here.
> > >
> > >So I was considering a 16K SRAM (128x128 = 16K).  Does anyone have any suggestions?? Would this be easy enough to implement? Ie connect my 4bit adc on port A, read the whole thing and write it parallel via port B to an SRAM.. that should take less than my 5 allowed instructions =)  I hope!! unless the SRAM is really complicated.. but I hope not..  I only really know of SRAM and DRAM, but I know that DRAM will not do the trick =) No chance to refresh anything here!
> > >
>
> Is my math wrong? The instruction clock at 20 Mhz is 5 MIPS. So each
> instruction cycle is 200ns. Right? 500Ksamples occurs at a rate of 1 every
> 20 uS. So 20 uS/.2 uS -> 100. So there's actually 100 instructions available
> between each sample.
>
> Did I miss something?

Duh. I missed a zero. 500KSamples is one every 2 uS. So there are 10
instructions inbetween each sample. That should be enough time to transfer the
data directly from the A/D to the ram, restart the ADC, and increment the RAM
address.

Does the A/D have a Data valid output? You may be able to use it to autowrite
to the RAM....

BAJ

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